An idling stop system, which stops an engine when a vehicle is stationary and restarts the engine when a driver resumes driving, has been put into practice considering an environmental issue. When the engine is restarted, because a starting current of a starter motor is large, a voltage of a battery reduces. As a result, lightings and the like are darkened temporarily. Further, when the engine is restarted in an unfavorable condition, such as the battery is deteriorated or the vehicle is in a cold district, the voltage of the battery will be lower than a voltage to operate safety devices and in-vehicle electric devices, such as navigation system and audio. In such a case, these devices may be unintentionally reset.
In an electric device or a control unit coupled to a battery, it has been proposed to provide a back-up power source in addition to a main power source that is supplied with electric power from the battery in order to address the above matter. For example, JP2003-148310A, which corresponds to US2003/0042873A1, describes to charge a capacitor with electric power from a battery and to supply the electric power charged in the capacitor as a back-up power supply.
As another example, JP2004-328988A; which corresponds to US2006/0097577A1, describes to employ a back-up power source such as a lithium ion battery, in addition to a main power source such as a lead battery, and to charge both of the main power source and the back-up power source with electric power generated by a generator such as an alternator. In JP2004-328988A, a main power source line and a back-up power source line are separately connected to a safety electronic control unit (ECU), and electric power is supplied to the safety ECU through a diode in a redundancy manner. Moreover, the back-up power source line is turned on when a predetermined period of time has elapsed after the main power source line is turned on.
In a unit including two power source lines as described above, if a malfunction occurs in the back-up power source, the malfunction of the back-up power source is easily detected by a detection circuit provided in the back-up power source. However, a voltage of a line at a cathode side of the diode forming the redundancy circuit is common between the main power source line and the back-up power source line. Therefore, it is difficult to detect an open fault of the diode.
Specifically, when the diode of the back-up power source line has the open fault, the voltage at the cathode side of the diode is increased by the voltage of the main power source line. Therefore, it is difficult to detect the open fault of the diode of the back-up power source line.
In a state where the diode of the back-up power source line has the open fault, if the engine is restarted from an idling stop state, there is a possibility that the voltage of the unit may be insufficient due to the unfavorable condition. For example, with regard to an airbag control unit for controlling activation of airbags according to a collision, the open fault of the diode of the back-up power source line needs to be properly detected.